[v4 PATCH 06/12] spi/atmel_spi: add dmaengine support

Richard Genoud
2013-01-23T16:25:09Z

2013/1/14 Wenyou Yang <wenyou.yang@atmel.com>:
> From: Nicolas Ferre <nicolas.ferre@atmel.com>
>
> Add dmaengine support.
>
> For different SoC, the "has_dma_support" is used to select
> the transfer mode: dmaengine or PDC.
>
> For the dmaengine transfer mode, if it fails to config dmaengine,
> or if the message len less than 16 bytes, it will use the PIO transfer mode.
>
> Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
> [wenyou.yang at atmel.com: using "has_dma_support" to select DMA as the spi xfer mode]
> [wenyou.yang at atmel.com: add support NPCS1,2,3 chip select other than NPCS0]
> [wenyou.yang at atmel.com: fix DMA: OOPS if buffer > 4096 bytes]
> Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
> Cc: grant.likely at secretlab.ca
> Cc: spi-devel-general at lists.sourceforge.net
> Cc: richard.genoud at gmail.com
> ---
> This patch is based on the original patch from Nicolas
> [PATCH] spi/atmel_spi: add dmaengine support
> and merge the patches from Richard Genoud <richard.genoud@gmail.com>
> [PATCH] spi-atmel: update with dmaengine interface
> [PATCH] spi-atmel: fix __init/__devinit sections mismatch
> and Wenyou Yang add the code to support selecting the spi transfer mode,
> add support NPCS1,2,3 chip select other than NPCS0.
> fix DMA: OOPS if buffer > 4096 bytes.
>
> diff --git a/drivers/spi/spi-atmel.c b/drivers/spi/spi-atmel.c
> index 43c1f63..0d242dc 100644
> --- a/drivers/spi/spi-atmel.c
> +++ b/drivers/spi/spi-atmel.c
> @@ -15,11 +15,13 @@
> #include <linux/platform_device.h>
> #include <linux/delay.h>
> #include <linux/dma-mapping.h>
> +#include <linux/dmaengine.h>
> #include <linux/err.h>
> #include <linux/interrupt.h>
> #include <linux/spi/spi.h>
> #include <linux/slab.h>
> #include <linux/platform_data/atmel.h>
> +#include <linux/platform_data/dma-atmel.h>
> #include <linux/of.h>
>
> #include <asm/io.h>
> @@ -182,6 +184,19 @@
> #define spi_writel(port,reg,value) \
> __raw_writel((value), (port)->regs + SPI_##reg)
>
> +/* use PIO for small transfers, avoiding DMA setup/teardown overhead and
> + * cache operations; better heuristics consider wordsize and bitrate.
> + */
> +#define DMA_MIN_BYTES 16
> +
> +struct atmel_spi_dma {
> + struct dma_chan *chan_rx;
> + struct dma_chan *chan_tx;
> + struct scatterlist sgrx;
> + struct scatterlist sgtx;
> + struct dma_async_tx_descriptor *data_desc_rx;
> + struct dma_async_tx_descriptor *data_desc_tx;
> +};
>
> /*
> * The core SPI transfer engine just talks to a register bank to set up
> @@ -192,6 +207,7 @@ struct atmel_spi_pdata {
> u8 version;
> bool has_dma_support;
> bool has_wdrbt;
> + struct at_dma_slave dma_slave;
> };
>
> struct atmel_spi {
> @@ -207,6 +223,7 @@ struct atmel_spi {
>
> u8 stopping;
> struct list_head queue;
> + struct tasklet_struct tasklet;
> struct spi_transfer *current_transfer;
> unsigned long current_remaining_bytes;
> struct spi_transfer *next_transfer;
> @@ -214,8 +231,15 @@ struct atmel_spi {
> int done_status;
> struct atmel_spi_pdata *pdata;
>
> + bool use_dma;
> + bool use_pdc;
> +
> + /* scratch buffer */
> void *buffer;
> dma_addr_t buffer_dma;
> +
> + /* dmaengine data */
> + struct atmel_spi_dma dma;
> };
>
> /* Controller-specific per-slave state */
> @@ -328,9 +352,7 @@ static bool atmel_spi_is_v2(struct atmel_spi *as)
> * and (c) will trigger that first erratum in some cases.
> *
> * TODO: Test if the atmel_spi_is_v2() branch below works on
> - * AT91RM9200 if we use some other register than CSR0. However, don't
> - * do this unconditionally since AP7000 has an errata where the BITS
> - * field in CSR0 overrides all other CSRs.
> + * AT91RM9200 if we use some other register than CSR0.
> */
>
> static void cs_activate(struct atmel_spi *as, struct spi_device *spi)
> @@ -340,18 +362,16 @@ static void cs_activate(struct atmel_spi *as, struct spi_device *spi)
> u32 mr;
>
> if (atmel_spi_is_v2(as)) {
> - /*
> - * Always use CSR0. This ensures that the clock
> - * switches to the correct idle polarity before we
> - * toggle the CS.
> - */
> - spi_writel(as, CSR0, asd->csr);
> + spi_writel(as, CSR0 + 4 * spi->chip_select, asd->csr);
>
> if (as->pdata->has_wdrbt)
> - spi_writel(as, MR, SPI_BF(PCS, 0x0e) | SPI_BIT(MSTR)
> - | SPI_BIT(MODFDIS) | SPI_BIT(WDRBT));
> + spi_writel(as, MR,
> + SPI_BF(PCS, ~(0x01 << spi->chip_select))
> + | SPI_BIT(MSTR) | SPI_BIT(MODFDIS)
> + | SPI_BIT(WDRBT));
> else
> - spi_writel(as, MR, SPI_BF(PCS, 0x0e)
> + spi_writel(as, MR,
> + SPI_BF(PCS, ~(0x01 << spi->chip_select))
> | SPI_BIT(MSTR) | SPI_BIT(MODFDIS));
>
> mr = spi_readl(as, MR);
> @@ -414,6 +434,12 @@ static void atmel_spi_unlock(struct atmel_spi *as)
> spin_unlock_irqrestore(&as->lock, as->flags);
> }
>
> +static inline bool atmel_spi_use_dma(struct atmel_spi *as,
> + struct spi_transfer *xfer)
> +{
> + return as->use_dma && xfer->len >= DMA_MIN_BYTES;
> +}
> +
> static inline int atmel_spi_xfer_is_last(struct spi_message *msg,
> struct spi_transfer *xfer)
> {
> @@ -425,6 +451,220 @@ static inline int atmel_spi_xfer_can_be_chained(struct spi_transfer *xfer)
> return xfer->delay_usecs == 0 && !xfer->cs_change;
> }
>
> +static bool filter(struct dma_chan *chan, void *slave)
> +{
> + struct at_dma_slave *sl = slave;
> +
> + if (sl->dma_dev == chan->device->dev) {
> + chan->private = sl;
> + return true;
> + } else {
> + return false;
> + }
> +}
> +
> +static int __devinit atmel_spi_configure_dma(struct atmel_spi *as)
> +{
> + struct at_dma_slave *sdata
> + = (struct at_dma_slave *)&as->pdata->dma_slave;
> +
> + if (sdata && sdata->dma_dev) {
> + dma_cap_mask_t mask;
> +
> + /* setup DMA addresses */
> + sdata->rx_reg = (dma_addr_t)as->phybase + SPI_RDR;
> + sdata->tx_reg = (dma_addr_t)as->phybase + SPI_TDR;
> +
> + /* Try to grab two DMA channels */
> + dma_cap_zero(mask);
> + dma_cap_set(DMA_SLAVE, mask);
> + as->dma.chan_tx = dma_request_channel(mask, filter, sdata);
> + if (as->dma.chan_tx)
> + as->dma.chan_rx =
> + dma_request_channel(mask, filter, sdata);
> + }
> + if (!as->dma.chan_rx || !as->dma.chan_tx) {
> + if (as->dma.chan_rx)
> + dma_release_channel(as->dma.chan_rx);
> + if (as->dma.chan_tx)
> + dma_release_channel(as->dma.chan_tx);
> + dev_err(&as->pdev->dev,
> + "DMA channel not available, unable to use SPI\n");
> + return -EBUSY;
> + }
> +
> + dev_info(&as->pdev->dev,
> + "Using %s (tx) and %s (rx) for DMA transfers\n",
> + dma_chan_name(as->dma.chan_tx),
> + dma_chan_name(as->dma.chan_rx));
> +
> + return 0;
> +}
> +
> +static void atmel_spi_stop_dma(struct atmel_spi *as)
> +{
> + if (as->dma.chan_rx)
> + as->dma.chan_rx->device->device_control(as->dma.chan_rx,
> + DMA_TERMINATE_ALL, 0);
> + if (as->dma.chan_tx)
> + as->dma.chan_tx->device->device_control(as->dma.chan_tx,
> + DMA_TERMINATE_ALL, 0);
> +}
> +
> +static void atmel_spi_release_dma(struct atmel_spi *as)
> +{
> + if (as->dma.chan_rx)
> + dma_release_channel(as->dma.chan_rx);
> + if (as->dma.chan_tx)
> + dma_release_channel(as->dma.chan_tx);
> +}
> +
> +/* This function is called by the DMA driver from tasklet context */
> +static void dma_callback(void *data)
> +{
> + struct spi_master *master = data;
> + struct atmel_spi *as = spi_master_get_devdata(master);
> +
> + /* trigger SPI tasklet */
> + tasklet_schedule(&as->tasklet);
> +}
> +
> +/*
> + * Next transfer using PIO.
> + * lock is held, spi tasklet is blocked
> + */
> +static void atmel_spi_next_xfer_pio(struct spi_master *master,
> + struct spi_transfer *xfer)
> +{
> + struct atmel_spi *as = spi_master_get_devdata(master);
> +
> + dev_vdbg(master->dev.parent, "atmel_spi_next_xfer_pio\n");
> +
> + as->current_remaining_bytes = xfer->len;
> +
> + /* Make sure data is not remaining in RDR */
> + spi_readl(as, RDR);
> + while (spi_readl(as, SR) & SPI_BIT(RDRF)) {
> + spi_readl(as, RDR);
> + cpu_relax();
> + }
> +
> + if (xfer->tx_buf)
> + spi_writel(as, TDR, *(u8 *)(xfer->tx_buf));
> + else
> + spi_writel(as, TDR, 0);
> +
> + dev_dbg(master->dev.parent,
> + " start pio xfer %p: len %u tx %p rx %p\n",
> + xfer, xfer->len, xfer->tx_buf, xfer->rx_buf);
> +
> + /* Enable relevant interrupts */
> + spi_writel(as, IER, SPI_BIT(RDRF) | SPI_BIT(OVRES));
> +}
> +
> +/*
> + * Submit next transfer for DMA.
> + * lock is held, spi tasklet is blocked
> + */
> +static int atmel_spi_next_xfer_dma_submit(struct spi_master *master,
> + struct spi_transfer *xfer,
> + u32 *plen)
> +{
> + struct atmel_spi *as = spi_master_get_devdata(master);
> + struct dma_chan *rxchan = as->dma.chan_rx;
> + struct dma_chan *txchan = as->dma.chan_tx;
> + struct dma_async_tx_descriptor *rxdesc;
> + struct dma_async_tx_descriptor *txdesc;
> + dma_cookie_t cookie;
> + u32 len = *plen;
> +
> + dev_vdbg(master->dev.parent, "atmel_spi_next_xfer_dma_submit\n");
> +
> + /* Check that the channels are available */
> + if (!rxchan || !txchan)
> + return -ENODEV;
> +
> + /* release lock for DMA operations */
> + atmel_spi_unlock(as);
> +
> + /* prepare the RX dma transfer */
> + sg_init_table(&as->dma.sgrx, 1);
> + if (xfer->rx_buf)
> + as->dma.sgrx.dma_address = xfer->rx_dma + xfer->len - *plen;
> + else {
> + as->dma.sgrx.dma_address = as->buffer_dma;
> + if (len > BUFFER_SIZE)
> + len = BUFFER_SIZE;
> + }
You should use brackets before the "else" also.
cf Documentation/CodingStyle line 169 :

This does not apply if only one branch of a conditional statement is a single
statement; in the latter case use braces in both branches:

if (condition) {
do_this();
do_that();
} else {
otherwise();
}

> +
> + /* prepare the TX dma transfer */
> + sg_init_table(&as->dma.sgtx, 1);
> + if (xfer->tx_buf) {
> + as->dma.sgtx.dma_address = xfer->tx_dma + xfer->len - *plen;
> + } else {
> + as->dma.sgtx.dma_address = as->buffer_dma;
> + if (len > BUFFER_SIZE)
> + len = BUFFER_SIZE;
> + memset(as->buffer, 0, len);
> + }
> +
> + sg_dma_len(&as->dma.sgtx) = len;
> + sg_dma_len(&as->dma.sgrx) = len;
> +
> + *plen = len;
> +
> + /* Send both scatterlists */
> + rxdesc = rxchan->device->device_prep_slave_sg(rxchan,
> + &as->dma.sgrx,
> + 1,
> + DMA_FROM_DEVICE,
> + DMA_PREP_INTERRUPT | DMA_CTRL_ACK,
> + NULL);
> + if (!rxdesc)
> + goto err_dma;
> +
> + txdesc = txchan->device->device_prep_slave_sg(txchan,
> + &as->dma.sgtx,
> + 1,
> + DMA_TO_DEVICE,
> + DMA_PREP_INTERRUPT | DMA_CTRL_ACK,
> + NULL);
> + if (!txdesc)
> + goto err_dma;
> +
> + dev_dbg(master->dev.parent,
> + " start dma xfer %p: len %u tx %p/%08x rx %p/%08x\n",
> + xfer, xfer->len, xfer->tx_buf, xfer->tx_dma,
> + xfer->rx_buf, xfer->rx_dma);
> +
> + /* Enable relevant interrupts */
> + spi_writel(as, IER, SPI_BIT(OVRES));
> +
> + /* Put the callback on the RX transfer only, that should finish last */
> + rxdesc->callback = dma_callback;
> + rxdesc->callback_param = master;
> +
> + /* Submit and fire RX and TX with TX last so we're ready to read! */
> + cookie = rxdesc->tx_submit(rxdesc);
> + if (dma_submit_error(cookie))
> + goto err_dma;
> + cookie = txdesc->tx_submit(txdesc);
> + if (dma_submit_error(cookie))
> + goto err_dma;
> + rxchan->device->device_issue_pending(rxchan);
> + txchan->device->device_issue_pending(txchan);
> +
> + /* take back lock */
> + atmel_spi_lock(as);
> + return 0;
> +
> +err_dma:
> + spi_writel(as, IDR, SPI_BIT(OVRES));
> + atmel_spi_stop_dma(as);
> + atmel_spi_lock(as);
> + return -ENOMEM;
> +}
> +
> static void atmel_spi_next_xfer_data(struct spi_master *master,
> struct spi_transfer *xfer,
> dma_addr_t *tx_dma,
> @@ -457,10 +697,10 @@ static void atmel_spi_next_xfer_data(struct spi_master *master,
> }
>
> /*
> - * Submit next transfer for DMA.
> + * Submit next transfer for PDC.
> * lock is held, spi irq is blocked
> */
> -static void atmel_spi_next_xfer(struct spi_master *master,
> +static void atmel_spi_next_xfer_pdc(struct spi_master *master,
> struct spi_message *msg)
> {
> struct atmel_spi *as = spi_master_get_devdata(master);
> @@ -557,6 +797,49 @@ static void atmel_spi_next_xfer(struct spi_master *master,
> spi_writel(as, PTCR, SPI_BIT(TXTEN) | SPI_BIT(RXTEN));
> }
>
> +/*
> + * Choose way to submit next transfer and start it.
> + * lock is held, spi tasklet is blocked
> + */
> +static void atmel_spi_next_xfer_dma(struct spi_master *master,
> + struct spi_message *msg)
> +{
> + struct atmel_spi *as = spi_master_get_devdata(master);
> + struct spi_transfer *xfer;
> + u32 remaining, len;
> +
> + dev_vdbg(&msg->spi->dev, "atmel_spi_next_xfer\n");
> +
> + remaining = as->current_remaining_bytes;
> + if (remaining) {
> + xfer = as->current_transfer;
> + len = remaining;
> + } else {
> + if (!as->current_transfer)
> + xfer = list_entry(msg->transfers.next,
> + struct spi_transfer, transfer_list);
> + else
> + xfer = list_entry(
> + as->current_transfer->transfer_list.next,
> + struct spi_transfer, transfer_list);
> +
> + as->current_transfer = xfer;
> + len = xfer->len;
> + }
> +
> + if (atmel_spi_use_dma(as, xfer)) {
> + u32 total = len;
> + if (!atmel_spi_next_xfer_dma_submit(master, xfer, &len)) {
> + as->current_remaining_bytes = total - len;
> + return;
> + } else
> + dev_err(&msg->spi->dev, "unable to use DMA, fallback to PIO\n");
same here.
> + }
> +
> + /* use PIO if error appened using DMA */
> + atmel_spi_next_xfer_pio(master, xfer);
> +}
> +
> static void atmel_spi_next_message(struct spi_master *master)
> {
> struct atmel_spi *as = spi_master_get_devdata(master);
> @@ -581,7 +864,10 @@ static void atmel_spi_next_message(struct spi_master *master)
> } else
> cs_activate(as, spi);
>
> - atmel_spi_next_xfer(master, msg);
> + if (as->use_pdc)
> + atmel_spi_next_xfer_pdc(master, msg);
> + else
> + atmel_spi_next_xfer_dma(master, msg);
> }
>
> /*
> @@ -634,6 +920,11 @@ static void atmel_spi_dma_unmap_xfer(struct spi_master *master,
> xfer->len, DMA_FROM_DEVICE);
> }
>
> +static void atmel_spi_disable_pdc_transfer(struct atmel_spi *as)
> +{
> + spi_writel(as, PTCR, SPI_BIT(RXTDIS) | SPI_BIT(TXTDIS));
> +}
> +
> static void
> atmel_spi_msg_done(struct spi_master *master, struct atmel_spi *as,
> struct spi_message *msg, int stay)
> @@ -659,19 +950,175 @@ atmel_spi_msg_done(struct spi_master *master, struct atmel_spi *as,
> as->done_status = 0;
>
> /* continue if needed */
> - if (list_empty(&as->queue) || as->stopping)
> - spi_writel(as, PTCR, SPI_BIT(RXTDIS) | SPI_BIT(TXTDIS));
> - else
> + if (list_empty(&as->queue) || as->stopping) {
> + if (as->use_pdc)
> + atmel_spi_disable_pdc_transfer(as);
> + } else
> atmel_spi_next_message(master);
same.
> }
>
> -static irqreturn_t
> -atmel_spi_interrupt(int irq, void *dev_id)
> +/* Called from IRQ
> + * lock is held
> + *
> + * Must update "current_remaining_bytes" to keep track of data
> + * to transfer.
> + */
> +static void
> +atmel_spi_pump_pio_data(struct atmel_spi *as, struct spi_transfer *xfer)
> {
> - struct spi_master *master = dev_id;
> + u8 *txp;
> + u8 *rxp;
> + unsigned long xfer_pos = xfer->len - as->current_remaining_bytes;
> +
> + if (xfer->rx_buf) {
> + rxp = ((u8 *)xfer->rx_buf) + xfer_pos;
> + *rxp = spi_readl(as, RDR);
> + } else {
> + spi_readl(as, RDR);
> + }
> +
> + as->current_remaining_bytes--;
> +
> + if (as->current_remaining_bytes) {
> + if (xfer->tx_buf) {
> + txp = ((u8 *)xfer->tx_buf) + xfer_pos + 1;
> + spi_writel(as, TDR, *txp);
> + } else {
> + spi_writel(as, TDR, 0);
> + }
> + }
> +}
> +
> +/* Tasklet
> + * Called from DMA callback + pio transfer and overrun IRQ.
> + */
> +static void atmel_spi_tasklet_func(unsigned long data)
> +{
> + struct spi_master *master = (struct spi_master *)data;
> struct atmel_spi *as = spi_master_get_devdata(master);
> struct spi_message *msg;
> struct spi_transfer *xfer;
> +
> + dev_vdbg(master->dev.parent, "atmel_spi_tasklet_func\n");
> +
> + atmel_spi_lock(as);
> +
> + xfer = as->current_transfer;
> +
> + if (xfer == NULL)
> + /* already been there */
> + goto tasklet_out;
> +
> + msg = list_entry(as->queue.next, struct spi_message, queue);
> +
> + if (as->current_remaining_bytes == 0) {
> + if (as->done_status < 0) {
> + /* error happened (overrun) */
> + if (atmel_spi_use_dma(as, xfer))
> + atmel_spi_stop_dma(as);
> + } else
> + /* only update length if no error */
> + msg->actual_length += xfer->len;
same
> +
> + if (atmel_spi_use_dma(as, xfer))
> + if (!msg->is_dma_mapped)
> + atmel_spi_dma_unmap_xfer(master, xfer);
> +
> + if (xfer->delay_usecs)
> + udelay(xfer->delay_usecs);
> +
> + if (atmel_spi_xfer_is_last(msg, xfer) || as->done_status < 0)
> + /* report completed (or erroneous) message */
> + atmel_spi_msg_done(master, as, msg, xfer->cs_change);
> + else {
same
> + if (xfer->cs_change) {
> + cs_deactivate(as, msg->spi);
> + udelay(1);
> + cs_activate(as, msg->spi);
> + }
> +
> + /*
> + * Not done yet. Submit the next transfer.
> + *
> + * FIXME handle protocol options for xfer
> + */
> + atmel_spi_next_xfer_dma(master, msg);
> + }
> + } else
same
> + /*
> + * Keep going, we still have data to send in
> + * the current transfer.
> + */
> + atmel_spi_next_xfer_dma(master, msg);
> +
> +tasklet_out:
> + atmel_spi_unlock(as);
> +}
> +
> +static int atmel_spi_interrupt_dma(struct atmel_spi *as,
> + struct spi_master *master)
> +{
> + u32 status, pending, imr;
> + struct spi_transfer *xfer;
> + int ret = IRQ_NONE;
> +
> + imr = spi_readl(as, IMR);
> + status = spi_readl(as, SR);
> + pending = status & imr;
> +
> + if (pending & SPI_BIT(OVRES)) {
> + ret = IRQ_HANDLED;
> + spi_writel(as, IDR, SPI_BIT(OVRES));
> + dev_warn(master->dev.parent, "overrun\n");
> +
> + /*
> + * When we get an overrun, we disregard the current
> + * transfer. Data will not be copied back from any
> + * bounce buffer and msg->actual_len will not be
> + * updated with the last xfer.
> + *
> + * We will also not process any remaning transfers in
> + * the message.
> + *
> + * All actions are done in tasklet with done_status indication
> + */
> + as->done_status = -EIO;
> + smp_wmb();
> +
> + /* Clear any overrun happening while cleaning up */
> + spi_readl(as, SR);
> +
> + tasklet_schedule(&as->tasklet);
> +
> + } else if (pending & SPI_BIT(RDRF)) {
> + atmel_spi_lock(as);
> +
> + if (as->current_remaining_bytes) {
> + ret = IRQ_HANDLED;
> + xfer = as->current_transfer;
> + atmel_spi_pump_pio_data(as, xfer);
> + if (!as->current_remaining_bytes) {
> + /* no more data to xfer, kick tasklet */
> + spi_writel(as, IDR, pending);
> + tasklet_schedule(&as->tasklet);
> + }
> + }
> +
> + atmel_spi_unlock(as);
> + } else {
> + WARN_ONCE(pending, "IRQ not handled, pending = %x\n", pending);
> + ret = IRQ_HANDLED;
> + spi_writel(as, IDR, pending);
> + }
> +
> + return ret;
> +}
> +
> +static int atmel_spi_interrupt_pdc(struct atmel_spi *as,
> + struct spi_master *master)
> +{
> + struct spi_message *msg;
> + struct spi_transfer *xfer;
> u32 status, pending, imr;
> int ret = IRQ_NONE;
>
> @@ -767,14 +1214,14 @@ atmel_spi_interrupt(int irq, void *dev_id)
> *
> * FIXME handle protocol options for xfer
> */
> - atmel_spi_next_xfer(master, msg);
> + atmel_spi_next_xfer_pdc(master, msg);
> }
> } else {
> /*
> * Keep going, we still have data to send in
> * the current transfer.
> */
> - atmel_spi_next_xfer(master, msg);
> + atmel_spi_next_xfer_pdc(master, msg);
> }
> }
>
> @@ -783,6 +1230,27 @@ atmel_spi_interrupt(int irq, void *dev_id)
> return ret;
> }
>
> +/* Interrupt
> + *
> + * No need for locking in this Interrupt handler: done_status is the
> + * only information modified. What we need is the update of this field
> + * before tasklet runs. This is ensured by using barrier.
> + */
> +static irqreturn_t
> +atmel_spi_interrupt(int irq, void *dev_id)
> +{
> + struct spi_master *master = dev_id;
> + struct atmel_spi *as = spi_master_get_devdata(master);
> + int ret;
> +
> + if (as->use_pdc)
> + ret = atmel_spi_interrupt_pdc(as, master);
> + else
> + ret = atmel_spi_interrupt_dma(as, master);
> +
> + return ret;
> +}
> +
> static int atmel_spi_setup(struct spi_device *spi)
> {
> struct atmel_spi *as;
> @@ -945,13 +1413,10 @@ static int atmel_spi_transfer(struct spi_device *spi, struct spi_message *msg)
>
> /*
> * DMA map early, for performance (empties dcache ASAP) and
> - * better fault reporting. This is a DMA-only driver.
> - *
> - * NOTE that if dma_unmap_single() ever starts to do work on
> - * platforms supported by this driver, we would need to clean
> - * up mappings for previously-mapped transfers.
> + * better fault reporting.
> */
> - if (!msg->is_dma_mapped) {
> + if ((!msg->is_dma_mapped) && (atmel_spi_use_dma(as, xfer)
> + || as->use_pdc)) {
> if (atmel_spi_dma_map_xfer(as, xfer) < 0)
> return -ENOMEM;
> }
> @@ -1068,6 +1533,8 @@ static int atmel_spi_probe(struct platform_device *pdev)
>
> spin_lock_init(&as->lock);
> INIT_LIST_HEAD(&as->queue);
> + tasklet_init(&as->tasklet, atmel_spi_tasklet_func,
> + (unsigned long)master);
> as->pdev = pdev;
> as->regs = ioremap(regs->start, resource_size(regs));
> if (!as->regs)
> @@ -1096,7 +1563,15 @@ static int atmel_spi_probe(struct platform_device *pdev)
> else
> spi_writel(as, MR, SPI_BIT(MSTR) | SPI_BIT(MODFDIS));
>
> - spi_writel(as, PTCR, SPI_BIT(RXTDIS) | SPI_BIT(TXTDIS));
> + as->use_dma = false;
> + as->use_pdc = false;
> +
> + if (as->pdata->has_dma_support) {
> + if (atmel_spi_configure_dma(as) == 0)
> + as->use_dma = true;
> + } else
> + as->use_pdc = true;
same.

Maybe you should add a little dev_info in case both use_dma and
use_pcd are null:
if (!as->use_dma && !as->use_pdc)
dev_info(&pdev->dev, "Atmel SPI Controller: Using PIO only for transferts");

I tested it on sam9g35.
It's working *almost* all right.
I've got a problem with the ioctl SPI_IOC_MESSAGE().
(cf Documentation/spi/spidev_fdx.c)
I'm reading on spidev something like that :
struct spi_ioc_transfer xfer[3];
memset(xfer, 0, sizeof(xfer));
xfer[0].tx_buf = (unsigned long)&tx_header;
xer[0].rx_buf = (unsigned long)&rx_header;
xfer[0].len = 2;
xfer[1].rx_buf = (unsigned long)rx_buf;
xfer[1].len = 124;
xfer[2].tx_buf = (unsigned long)&tx_footer;
xfer[2].rx_buf = (unsigned long)&rx_footer;
xfer[2].len = 2;

nb = ioctl(spi_data->fd, SPI_IOC_MESSAGE(3), xfer);

If I force DMA all the time with
#define DMA_MIN_BYTES 1
I haven't got any problem, all works ok.
If I for PIO all the time with
#define DMA_MIN_BYTES 1000
I'ts woking also without any problem.

BUT
When the DMA_MIN_BYTES are at 16 (so, xfer[0] is transfered by PIO,
xfer[1] by DMA and xfer[2] by PIO), I've got a lot of errors in my
rx_buffer.
ie, I'm not receiving what I should.

Did you experience anything like that ?

Regards,
Richard